Gasification of biomass, waste or coal produces synthesis gas, a gas essentially comprising methane, carbon monoxide, hydrogen, water and carbon dioxide. Synthesis gas is also known as syngas. Syngas may be used as a source of fuel, hydrogen or carbon monoxide, or may be used for downstream syngas conversions. Manipulation of reaction equilibria and reaction conditions influences the ratio of the gaseous products and therefore provides the preferred gas (i.e. methane, hydrogen or carbon monoxide). Apart from the aforementioned main constituents the product gas from a gasifier contains sulphur compounds, e.g. H2S and COS, ammonia, dust, alkali, impurities such as P, As, Hg etc. and tar components.
Gasification may be carried out above- or under-ground. Above-ground gasification of biomass, waste and coal may be carried out using an apparatus selected from the group consisting of moving bed reactor (Lurgi-type reactor), fluid bed apparatus, or a biomass gasifier, including, for example, a pyrolysis unit as described in Gasification by Christopher Higman and Maarten van der Burgt GPP, Elsevier Amsterdam 2008 and ‘Biomass Gasification’ chapter 4 in ‘Alternative Energy in Agriculture’ Vol. II, Ed. D. Yogi Goswami CRC Press 1986 pp 83-102. Underground coal gasification is described in http://www.ucgassociation.org
Gasification of biomass, waste and coal may be carried out at temperatures of up to 1800° C. [Higman and Maarten van der Burgt]. The temperature of the gasification of biomass, waste or coal may also be 1000° C. or less. The temperature of the gasification of biomass, waste or coal may be 900° C. or less, 800° C. or less, less than 800° C., 750° C. or less, less than 750° C., 700° C. or less, less than 700° C. The product gas stream has a pressure of between 1 barg and 100 barg, between 2 barg and 100 barg, up to 100 barg, up to 25 barg, up to 20 barg. Barg means overpressure in bar. Typically the biomass gasifier has an operating pressure from 0-20 barg.
Biomass & Bioenergy 24 (2003) pp 125-140 discloses that the temperature of such gasification processes not only affects the amount of tar formed, but also the composition of the tar. Tar composition ranges from primarily oxygenates at lower gasification temperatures (e.g. alcohols, aldehydes, ethers, esters, ketones, organic acids, and sugars), to primarily poly-aromatic hydrocarbons at higher gasification temperatures. Besides tar, the gasifier syngas also contains lighter hydrocarbons, e.g. methane, ethene, ethane, propene, propane, benzene and toluene.
Steam reforming of tar, or tar reforming, is the process carried out over a catalyst to convert tar to syngas. The process of reducing the tar species present in syngas is part of the gas conditioning, i.e. the train of unit operations needed before the syngas may be utilised for downstream applications, such as synthesis of chemicals or electricity. Dust in the syngas may be removed upstream or downstream of the tar reforming step, corresponding to the embodiments of tar reforming taking place in nearly dust free or dusty environment. The term “clean tar reformer” is meant to denote a tar reformer in a biomass gasification system wherein a filter is present between a biomass gasification unit and the tar reformer. This is opposed to the term “dusty tar reformer” which is meant to denote a tar reformer in a biomass gasification system wherein no filter, besides possible cyclones for a coarse removal of dust, is present between the biomass gasification unit and the tar reformer. The catalysts may be present in pellet or monolith form, of which the monoliths and pellets may be used for the clean tar reformer, whereas monoliths must be applied for the dusty tar reformer. The process of the present invention is suitable both for use in a dusty or dust-free environment, here referred to as dusty tar reforming and clean tar reforming, respectively.
In a dust-free environment, carbon coke formation from decomposition of syngas tar and hydrocarbons can take place in and on the catalyst, and/or carbon soot generated from partial combustion of the syngas from the gasifier can deposit and build up in and on the catalyst. This can lead to catalyst deactivation and/or increased pressure drop and inhomogeneous gas distribution due to locally increased restriction of gas passage through the catalyst volume. These phenomena all affect catalyst performance negatively. In addition, in a dusty environment the dust entrained in the gas can itself build-up in and on the catalyst, and lead to worsened, i.e. reduced, catalyst performance similar to the dust-free case. The dust has high carbon content, e.g. about 70% by weight, and is typically of a pyrolytic nature. Dust, coke, and soot, are all herein commonly referred to as carbonaceous materials.
Regeneration procedures for catalysts within a tar reformer are essential to ensure prolonged catalyst lifetime. Tar reformer catalyst regeneration is however challenging in relation to the risk of excessive temperature development within the reformer as a result of the exothermic carbon combustion, as well as energy consumption. Furthermore, pyrolytic and graphitic type of carbon from hydrocarbon decomposition, soot formation, or dust has low reactivity and can require temperatures above 600° C. for efficient oxidative combustion removal. The amounts of the deposited carbon can be of such large quantities that it requires a very controlled high-temperature oxidative burn-off to avoid temperature run-aways.
Regeneration of a catalyst within a tar reformer may be a process with very high energy consumption and/or with an excessive consumption of steam. US2012/0058030 discloses a method for regenerating catalyst for reforming tar-containing gas. US2012/0058030 describes that in a method for regenerating a deteriorated catalyst, water vapour is fed in a catalyst reactor, where water vapour reacts with carbon to remove carbon present on the surface of catalyst, or water vapour reacts with sulphur to remove sulphur adsorbed thereon and thereby regenerates the catalyst. Air may be incorporated instead of a part or entirety of water vapour.